The present invention relates to a light beam scanning apparatus applicable to an image forming apparatus of laser beam printers, digital photocopiers, plate making machines and the like.
A light beam scanning apparatus irradiates a light beam such as a laser beam or the like onto a photosensitive body. The light beam is modulated in accordance with image information signals. The modulated light beam is scanned across a predetermined range of the photosensitive body and in a fixed direction. This is the main scan. Then, the photosensitive body is moved at a constant speed in a direction perpendicular to the main scan so that a sub scan can be performed with respect to the photosensitive body by the light beam. By having a main scan and a sub scan over the photosensitive body by the light beam, a latent image such as an electrostatic latent image, corresponding to the image information signal is formed on the photosensitive body.
FIG. 1 indicates a general structure of a laser beam printer to which a laser beam scanning apparatus is applied.
A photosensitive layer is formed on a surface of a photosensitive drum which rotates at a constant speed. The laser beam emitted from the laser beam scan unit 3 scans in the direction of the axis of rotation. A charge unit 2, a developer 4, a transfer unit 5 that transfers the toner image formed on the surface of the photosensitive drum 1, to a recording sheet 7, and a cleaning unit 6 in order to remove the toner that adheres to the surface of the photosensitive drum 1 are disposed around the photosensitive drum 1. The surface of the charged photosensitive drum 1 is scanned by the laser beam modulated in accordance with image information signal from the laser scan unit so that an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 1. The electrostatic latent image is developed into a toner image by a developer 4 and this toner image is transferred by the transfer unit 5 to the recording sheet 7 transported at the same speed as the periphery of the photosensitive drum 1. Then, in the process whereby the recording sheet 7 is transported, the toner image formed on the recording sheet 7 is heated and has pressure applied to it by a fixer unit 8 so that the toner image is fixed to the recording sheet 7.
The structure of the laser beam scan unit 3 described above is indicated in FIG. 1 and FIG. 3.
Inside a closed case 9 are housed a laser light source 10, a cylinder lens 11, a polygon mirror 12 that rotates at a predetermined speed, an f-.theta. lens 13 and a mirror 14. The bottom of the closed case 9 opposite the photosensitive drum 1 has formed in it an opening 15. This opening 15 of the closed case 9 is covered by a glass plate 16. The glass plate 16 prevents the entry of toner suspended in the air and the like from passing through the opening 15 into the closed case 9.
FIG. 3 indicates the structure of the optical system in the laser beam scan unit 3 in more detail.
The laser light source 10 has a semiconductor laser element 17 and a collimator lens 18. The light beam emitted from the semiconductor laser element 17 modulated in accordance with the image information signals is made into a parallel beam by the collimator lens 18. The laser beam that passes through the collimator lens 18 then passes through a cylinder lens 11 and is successively irradiated onto each of the surfaces of the polygon mirror 12 that is rotating at high speed. The laser beam irradiated to each of the surface of the polygon mirror 12 is deflected by a predetermined angle in accordance with the rotation of the polygon mirror 12. The deflected laser beam passes through the f-.theta. lens 13, is reflected by the mirror 14, and passes through the glass plate 16 to be focussed onto the surface 1a of the photosensitive drum 1. When the laser beam is deflected by a predetermined angle by the rotation of the polygon mirror 12, the point of focus of the laser beam on the surface 1a of the photosensitive drum 1 shifts in a predetermined direction in a straight line parallel to the axis of rotation of the photosensitive drum 1. This shifting of the point of focus of the laser beam performs the main scan. The speed of the shift of focus of the laser beam is a constant speed determined by the f-.theta. characteristics of the f-.theta. lens 13. In order for the photosensitive drum 1 to rotate at a fixed speed, the point of focus of the laser beam on the surface 1a of the photosensitive drum 1 shifts at a constant speed in the direction of the periphery of the photosensitive drum 1. This point of focus performs the sub scan by shifting in the direction of the periphery of the photosensitive drum 1. On the basis of the image information signals, the modulated laser beam performs a main scan and a sub scan over the surface 1a of the photosensitive drum 1 so that an electrostatic latent image corresponding to the image information is formed on the surface 1a of the photosensitive drum 1.
The beam at both ends of the range of deflection of the laser beam does not contribute to the formation of the electrostatic latent image corresponding to the image information. The laser beam at these end portions is reflected by mirror 13a and is irradiated to a light-receiving element 20. The signal output from the light-receiving element 20 are used as synchronization signal for aligning the start position for the main scan. As is indicated in FIG. 2, the light-receiving element 20 and the laser light source 10 are fixed to the side wall of the closed case 9.
As has been explained above, the track of the point of focus of the laser beam on the surface 1a of the photosensitive drum 1, that is to say, the scan line, is ideally a straight line parallel to the axis of rotation of the photosensitive drum 1. In order to make the scan line an accurate and straight line, the optical axes of each of the lenses, each of the mirrors of the polygon mirror and the other mirrors and the like in the configuration of the optical system must be accurately held at their ideal statuses. For example, if the accuracy of processing of the lens, the accuracy of processing of the holder that holds the lens, and the accuracy of mounting them are poor, then the positions of said lens will deviate (in the direction of the sub scan, for example) from its ideal position and the main scan line will bend in the direction of the sub scan.
If the position of the cylinder lens 11 and the f-.theta. lens 13 described above are ideal as is indicated by the solid line in FIG. 4, then the scan line on the surface 1a of the photosensitive drum 1 will be a straight line as is indicated by (a) in FIG. 5. However, if the cylinder lens 11 inclines or the f-.theta. lens 13 deviates in the upwards direction as indicated by the broken line in FIG. 4, then the optical axes of each of the lenses will deviate from their ideal statuses and the scan line on the surface 1a of the photosensitive drum 1 will bend in a curve in the direction of the sub scan as indicated by (b) in FIG. 5.
If the scan line bends in a curve, then the image that is formed will have the straight lines represented as curves. Accordingly, if the scan lines are curved, then high-accuracy printers and color printers that form a color image by overlapping a plural number of monochrome images formed using a plural number of laser beam scanning units will have image distortion, non-correspondence of colors and other factors that will degrade the quality.